The human heart contains four valves: the tricuspid valve, the pulmonic valve, the mitral valve and the aortic valve. Proper functioning of each of these valves is essential to good health. A variety of pathologies, such as congenital defects, endocarditis, and rheumatic fever, can lead to dysfunction of one or more heart valves and, ultimately, to heart failure. Recent statistics show that each year, in the United States alone, valvular heart disease is responsible for nearly 20,000 deaths and is a contributing factor in approximately 42,000 deaths.
Surgical replacement of a diseased human heart valve with a prosthetic valve was first performed successfully in 1960. The procedure is now common, with approximately 300,000 patients worldwide undergoing heart valve replacement surgery each year. For the vast majority of patients, open heart surgical valve procedures provide an established form of therapy with reasonable risk and are proven to have long-term benefits.
The first practical experiment utilizing a minimally invasive, transcatheter heart valve (THV) was reported in 1992. Investigators fabricated a stent-mounted bioprosthetic valve and implanted it via balloon catheter, which demonstrated the feasibility of percutaneous implantation. The use of a stent-mounted bioprosthesis for pulmonic valve replacement was pioneered in 2000. Using a bovine jugular vein valve mounted within a stent, the first in-human percutaneous implantations of artificial valves in children with right ventricle to pulmonary prosthetic conduits were performed. This achievement clearly marked the beginning of the era of percutaneous valve replacement therapy in patients. The first in-human percutaneous aortic valve replacement was reported in 2002 on a 57-year-old man with inoperable critical aortic stenosis. Recently, in November 2011, U.S. FDA approved the Edwards SAPIEN transcatheter valve for use in the U.S. market.
Currently, transcatheter valve replacement holds promise for a large number of patients who otherwise have limited or no treatment options. However, it also poses various challenges due to its unique disease treatment mechanism. The transcatheter valve technique relies at least partially upon a frictional type of engagement between an expanded stent structure and the native tissue to maintain the position of the transcatheter valve for its normal function. The transcatheter valve stent can become partially embedded in the valve tissue during radial expansion. Improper host-implant interactions can lead to various dangerous events for the patient. For instance, excessive radial force from transcatheter aortic valve stent expansion may cause injury to the aorta, while insufficient force may lead to paravalvular leakage and device migration.
For patients with valve stenosis, heavy calcium deposition on the valve leaflets and on the valve root can also cause a distortion of transcatheter valve geometries, resulting in a valve of an elliptical shape instead of a nominal circular shape. Recent studies indicate that transcatheter aortic valve frames often do not reach their nominal designed dimensions but instead undergo asymmetric or non-circular expansion. An elliptical transcatheter valve configuration can result in negative clinical consequences, such as affecting leaflet coaptation, which can cause valve regurgitation. Furthermore, without proper leaflet apposition, uneven distribution of stress on the leaflets may affect a valve's long-term performance and durability.
Currently, transcatheter aortic valve replacement is not recommended for bicuspid aortic valve (BAV) patients because of the elliptical shape of the BAV and a high possibility of paravalvular leak caused by the gap between a circular transcatheter valve and an elliptical BAV shape. Recently, it has been found that a distorted, elliptical transcatheter aortic valve induces a significant increase in the leaflet peak stresses (143% at the eccentricity of 0.68) and strains (59% at the eccentricity of 0.68) compared with the nominal circular TAV under the same boundary/loading conditions.
An artificial heart valve that employs a leaflet design, whether a transcatheter heart valve or a surgical bioprosthetic heart valve, is susceptible to wear and tear of the leaflet material, due to normal valve cycling over prolonged periods of time. Studies have shown that the regions of tearing of bioprosthetic heart valve correlate with the regions of high tensile and bending stresses acting on the leaflets during opening and closing. Stress concentrations within the cusp can either directly accelerate tissue structural fatigue damage, or initiate calcification by causing structural disintegration, enabling multiple pathways of calcification that can lead to valve failure.
Thus, what is needed in the art is a prosthetic valve device that reduces leaflet stress and accommodates elliptical deformation of the transcatheter valve after implantation.